1. Field of the Invention
This invention relates to a method and apparatus for the guidance of the advance of a rotary drillstring, and more particularly to a method and apparatus for maintaining or controlling the trajectory of a rotating drill bit by modulating the :thrust on the drill bit in synchronization with the rotation of the drill rod. The present invention is contemplated for use in coal mine drilling and in oil field directional drilling.
The primary factors affecting the direction of rotary drilling are drill bit thrust, or weight-on-bit (WOB), and the rotational speed of the drill bit. Generally, with regard to a horizontal drillstring, increasing the thrust and increasing the rotational speed tend to cause a downward effect on the trajectory, while increasing the thrust and reducing the rotational speed tend to cause an upward trajectory.
2. The Prior Art
Heretofore, there have been several approaches taken to maintain a rotary drill bit trajectory along a desired path and upon which the present invention improves. It is known that positioning a stabilizer or centralizer on the drill rod near the drill bit increases the tendency of the bit to move upwardly, and positioning the stabilizer a greater distance behind the bit tends to cause a downward trajectory of the bit. Using prior art procedures, it was necessary, upon encountering a downward dip in a coal bed, for example, to remove the drillstring from the hole and remove or add a stabilizer adjacent the bit. It is also known that, in a horizontal bore, the drill bit will turn downward when there is low thrust and no hard interface below the drillstring, and it will turn upward when there is high thrust and no hard interface or layer above the drillstring.
In another procedure, directional drilling is achieved by inserting, at the downhole end of a drillstring, a small section of pipe called a sub which has been bent, i.e., a bent-sub, such that the longitudinal axis of one of its ends is at a slight angle to the other end. In practice, a borehole is drilled to a predetermined length and the drillstring is then withdrawn and a bent-sub having the desired offset angle is inserted between the end of the drillstring and a downhole motor. The drillstring is then inserted back into the borehole and, since the longitudinal axis of the drill bit is then at an angle to the original borehole due to the bent-sub, the direction of the borehole is altered. The bent-sub may be replaced any number of times in order to provide a borehole of the desired shape and configuration. U.S. Pat. No. 4,697,651 to Thomas B. Dellinger discloses such a bent-sub with a downhole drill motor and a monitoring device.
The use of downhole motors, however, tends to increase the cost of any given drilling operation due to the significant chance of loosing a drillstring. With such a loss, the cost of both the downhole motor and the instrument package would be incurred.
Another method of guiding the drill bit along the designated path is by means of a deflection operation carried out at a second location spaced longitudinally along the drillstring from the drill bit. This deflection operation involves repeatedly deflecting the drillstring from its axis in a radial direction during rotation of the drillstring. This guidance system comprises a segment member adapted to be inserted into the bore hole as a portion of the drillstring. This segment is provided with deflectors which are cyclically actuated between projected and retracted positions to change direction of the drill bit. Examples of this are disclosed in U.S. Pat. Nos. 4,461,349 and 4,471,843 to Emrys H. Jones, Jr. and Ronald W. Umphrey, and British patent application No. 2,066,878 to Heinz Wallussek et al. U.S. Pat. No. 4.305,474 to Nathandale Farris et al. discloses a guidance system with deflectors that also includes a downhole motor in which the deflectors are actuated when a lower than normal thrust is imposed on the drillstring and an opposite perpendicular force is exerted when a higher than normal thrust is imposed on the drillstring. When normal thrust is being used, the device does not cause deflection in either direction.
It is also known to provide monitoring devices to control the trajectory while drilling as disclosed, for example, in U.S. Pat. No. 4,733,733 to William B. Bradley et al., and in U.S. Pat. No. 4,471,843 to Jones et al., mentioned above.